1. Field of the Invention
The invention relates to a drive control device for a vehicle and, more particularly, to a technique for, in the vehicle that is able to travel in coasting, that is, in a state where engine braking force is decreased as compared to that in engine braking traveling, achieving both fuel economy and drivability of the vehicle during coasting on an upward-gradient road surface.
2. Description of Related Art
There is engine braking traveling in which a vehicle travels while engine braking is being applied through driven rotation of an engine in a state where a power transmission path between the engine and drive wheels remains connected. In contrast to this, there is conceivably coasting in which a vehicle travels while engine braking force is decreased as compared to that in the engine braking traveling in order to contribute to improvement in fuel economy by extending a travel distance. One example is a device described in Japanese Patent Application Publication No. 2002-227885 (JP 2002-227885 A). The device is configured to, when it is determined that an accelerator pedal has been returned while the vehicle is traveling, the vehicle is caused to start coasting by releasing a clutch provided in a power transmission path between an engine and drive wheels. Thus, the fuel economy of the vehicle is improved.
When the vehicle is coasting on an upward-gradient road surface, a vehicle speed easily decreases and the need for re-acceleration increases as compared to when the vehicle is coasting on a flat road or a downward-gradient road surface. Incidentally, the coasting of the vehicle conceivably includes free-run coasting in which the engine is disconnected from the drive wheels by releasing the clutch in the power transmission path and rotation of the engine is stopped by stopping supply of fuel to the engine. The coasting of the vehicle conceivably also includes neutral coasting in which the engine is operated by being supplied with fuel in a state where the engine is disconnected from the drive wheels by releasing the clutch in the power transmission path. These coastings differ from each other as follows in control at the time of returning to normal traveling. The engine is stopped in the free-run coasting, so it is required to restart the engine at the time of returning to the normal traveling and then to engage the clutch. On the other hand, the engine is rotating in the neutral coasting, so it is just required to engage the clutch at the time of returning to the normal traveling.
However, JP 2002-227885 A does not consider how a stop condition based on an upward gradient of a road surface is set for each of the free-run coasting and the neutral coasting. Therefore, it is not possible to appropriately stop coasting on the basis of the upward gradient of the road surface, and there may occur the following inconvenience. That is, if a relatively large gradient is set as an upper limit value of the upward gradient of the road surface at which coasting is stopped, when the vehicle is travelling in the free-run coasting at a place where the upward gradient of the road surface is relatively large and the likelihood of re-acceleration is high, the engine is started at the time of returning from the coasting to the normal traveling because the engine is stopped. Therefore, the response of re-acceleration deteriorates as compared to when the vehicle travels in the neutral coasting. On the other hand, if a relatively small gradient is set as the upper limit value of the upward gradient of the road surface at which the coasting is stopped, when the vehicle is traveling in the neutral coasting at a place where the upward gradient of the road surface is relatively small and the likelihood of re-acceleration is low, there is inconvenience that the fuel economy of the vehicle deteriorates because the engine remains operated.